This invention relates, in general, to adjustable gain differential amplifiers, and more specifically, to an adjustable gain differential amplifier which supplies high gain having high stability over a wide bandwidth.
Designers of differential amplifiers recognize that the gain of the amplifiers is not readily adjusted to a fixed value. To obtain a fixed gain and preferred frequency characteristics, the differential amplifier is commonly coupled to a feedback circuit, or to another device which supplies the desired gain. For example, U.S. Pat. No. 4,881,044 issued Nov. 14, 1991 to Kinoshita, et al., teaches a device having a current source supplying a current bias coupled in parallel with a load resistor of a differential amplifier. The gain is set by increasing the transconductance of the current source.
Even with the device taught in the '044 patent, the gain is not readily altered, and cannot independently be set at a desired gain. Since each current source and load resistor posses different temperature characteristics (i.e. coefficients of expansion) and are process dependent, temperature changes and process changes can result in gain fluctuations.